Conventionally, a magnetron is normally used in a portion for oscillating microwaves in a microwave oven. As is well known, in the magnetron, a direct current high voltage (3800 to 4000 V) is applied to an anode as is well known, and a constant voltage (about 4 V) as a heater voltage is applied to a filament for emitting electrons to the anode.
When a commercial power source voltage is increased to generate the direct current high voltage by a transformer in which silicon steel plates, etc. are stacked with each other to form a core and is rectified by a rectifier, the outer shape and weight of the transformer are greatly increased as the capacity of the transformer increases since the frequency is low.
Therefore, a dust core such as ferrite core having less electric loss by a high frequency electric current is used without using silicon steel in the transformer for driving the magnetron at the present time. A direct current voltage provided by rectifying a commercial electric source is converted to a high frequency power by an inverter device and is then inputted to a primary winding of the transformer and thereafter is increased to a several thousand voltage. Then, the increased voltage is rectified by a rectifier and is applied to an anode of the magnetron.
FIG. 1 shows a circuit diagram of a conventional apparatus for driving a magnetron using a direct current high voltage circuit. In this figure, a smoothing circuit 4A is composed of a choke coil 3 and a smoothing capacitor 4 and is connected in series between an anode and a cathode of a rectifing stack 2 for rectifing a commercial power source 1. One terminal of a primary winding 5a of a step-up transformer 5 is connected to a connection point between the smoothing capacitor 4 and the choke coil 3, and the other terminal of the primary winding 5a is connected to a cathode line through a resonant capacitor 6. Both terminals of the resonant capacitor 6 are connected in parallel to a free-wheeling diode 7 and collector and emitter of a transistor 8, and the base of the transistor 8 is connected to a control circuit 9 for controlling the switching operation of the transistor. The primary winding 5a, the resonant capacitor 6, the free-wheeling diode 7 and the transistor 8 form a quasi-E class inverter.
A voltage doubler rectifing circuit 11A is connected in parallel to a secondary winding 5b of the step-up transformer 5 and is composed of a diode 10 and a capacitor 11 connected in series to each other and controlling the charge and discharge of the secondary winding. A rectified output from both terminals of the diode 10 is applied between an anode A and a filament F of a magnetron 12. An alternating voltage as a heater voltage is supplied by a heater transformer 13 from the side of the commercial power source 1 to the filament F.
The operation of the conventional magnetron drive apparatus constructed above will be described next. First, when an alternating voltage is applied to the rectifing stack 2, all the alternating voltage waveforms are rectified and thereafter are smoothed by the smoothing circuit 4A composed of the choke coil 3 and the smoothing capacitor 4, and are then inputted to the direct current resonant circuit of the inverter composed of the primary winding 5a of the step-up transformer 5 and the resonant capacitor 6. With respect to the rectified input, when the transistor 8 is turned on by the control circuit 9, a primary electric current in the forward direction flows through the primary winding 5a and the transistor 8, and when the transistor 8 is turned off, the primary winding 5a and the resonant capacity 6 cause a series resonant state so that the voltage of the primary winding 5a is changed, generating a high frequency magnetic field by the repetition of this state. The high frequency power by the change of this magnetic field is increased to about 2000 V in accordance with the winding ratio of the step-up transformer 5, and thereafter becomes a rectified output at about 3800 to 4000 V by the voltage doubler rectifing circuit 11A composed of the diode 10 and the capacitor 11, and is then applied to the anode A of the magnetron 12. A voltage is supplied to the heater of the filament F of the magnetron 12 by another transformer 13 for the heater different from the step-up transformer 5, oscillating a microwave from the magnetron 12 by emitting electrons from the filament F.
However, in the conventional apparatus mentioned above, the transformer 13 for the heater is used separately from the step-up transformer 5, and is used for commercial frequency, thereby relatively increasing the weight thereof. Accordingly, a compact and light apparatus cannot be sufficiently provided in spite of using a dust core such as ferrite core for the step-up transformer 5 and forming the high voltage direct current power source circuit like an inverter. Further, the design of space for the apparatus is restricted and it is necessary to dispose two transformers, increasing the cost of the entire apparatus.
To solve the problems mentioned above, another conventional apparatus has been proposed as shown in FIG. 2. In this apparatus, a heater of a magnetron 12 is heated by a high frequency power outputted from a secondary winding 51c of a step-up transformer 51. The constitutional example of the step-up transformer 51 in this case is illustrated in FIG. 3. In the step-up transformer 51, a primary winding 51a and a secondary winding 51b are concentrically wound through a core 51e, and the secondary winding 51b on the high voltage side can be easily insulated by a bobbin 51f of the primary winding 51a, which is advantageous.
However, in the case of the step-up transformer 51, the coupling coefficient between the primary and secondary windings is increased so that the input inductance of the step-up transformer 51 is reduced and the oscillating frequency of the inverter becomes too high, providing no desirable output. To solve these problems, a coil 51d is connected in series to a capacitor 9' constituting a voltage doubler rectifing circuit to compensate the above inductance. However, as a result, it is necessary to dispose a separate coil although the transformer is a single unit so that the problems similar to those in the conventional apparatus provided with two transformers remain left.
The above description relates to the secondary winding for supplying power to the filament F, but it is necessary to reduce the variation in power supplied to the filament F as small as possible so as to stably operate the magnetron as characteristics of the magnetron. When the power supplied to the filament F is low, the amount of electrons emitted from the filament F, i.e., the emitted electrons therefrom become insufficient, resulting in molding in which the magnetron is unstably oscillated. When the supplied power is excessive, an excessive electric current flows through the filament F, damaging the filament and the magnetron and greatly reducing the life thereof. In general, it is necessary to be able to operate a microwave oven even when the input power source voltage of the magnetron drive apparatus is fluctuated by .+-.10% of a rated voltage due to great variation in power demand. The variation in power supplied to the filament F depending on the variation of the input power source voltage must be restricted to output from the magnetron a stable microwave power with respect to the variation of the input power source voltage. Further, with respect to the characteristics of the magnetron, when the magnetron is operated continuously, the oscillating voltage of the magnetron is reduced by the increase in temperature of the magnetron itself. Thus, it is necessary to secure small variation in power supplied to the filament even when the load is varied.
As mentioned above, in the conventional drive apparatus of the magnetron constructed as above, it is necessary to dispose another transformation for supplying power to the filament of the magnetron in addition to the step-up transformer for supplying power to the anode of the magnetron, and it is necessary to connect a coil for compensating the input inductance onto the secondary winding side of the transformer even when the step-up transformer and the transformer for supplying the heater voltage are integral with each other, thereby increasing the cost of the entire apparatus. Further, the advantages that the magnetron drive apparatus is made compact and light are lost by using the inverter power source apparatus. Further, it is necessary to dispose a means for restricting the variation in power supplied to the filament with respect to the fluctuation of about .+-.10% of the power source voltage.